One or more power generation systems, such as wind turbines, may be connected to a utility grid to supply electric energy to the utility grid. On the other hand, one or more consumers or loads are connected to the utility grid to extract electric energy from the utility grid. The utility grid may deliver the electric energy in form of a AC power stream (or signal or electromagnetic wave) which have a predetermined nominal grid frequency, such as 50 Hz or 60 Hz. Thereby, the grid frequency may highly depend on the balance of generated and consumed power. This balance of generated and consumed power is necessary to keep the frequency stable, but due to outage, generation loss and sudden increase in power a variation in frequency is often observed.
U.S. Pat. No. 7,345,373 B2 discloses a system and method for utility and wind turbine control, wherein a flow of power through the converter is modulated in response to frequency disturbances or power swings of the utility system relative to an internal reference frame which is implemented as an integrator that emulates a virtual inertia with a particular magnitude defined by the constant M. Thereby, the internal reference frame has an output that is variable and is the frequency of the internal reference frame. A relative frequency is obtained as a difference of a measured frequency (measured utility system frequency) and the frequency of the internal reference frame. In particular, the frequency of the internal reference frame may differ from the utility system during frequency disturbances.